A generalised model of forest productivity using simplified concepts of radiation-use efficiency, carbon balance and partitioning

Abstract

This paper describes a stand growth model, based on physiological processes, which incorporates a number of steps and procedures that have allowed considerable simplification relative to extant process based models. The model, called 3-PG (use of Physiological Principles in Predicting Growth), calculates total carbon fixed (gross primary production; P(G)) from utilizable, absorbed photosynthetically active radiation (φ(p.a.u)), obtained by correcting the photosynthetically active radiation absorbed by the forest canopy (φ(p.a.)) for the effects of soil drought, atmospheric vapour pressure deficits and stand age. P(G) is obtained from φ(p.a.u) and the canopy quantum efficiency, values of which are becoming available. The ratio of net (P(N)) to gross primary production is emerging as relatively constant for trees. This eliminates the need to calculate respiration and is used to estimate P(N)-the net amount of carbon converted to biomass. 3-PG uses a simple relationship to estimate the amount of carbon allocated below ground and a procedure based on allometric ratios-widely available for many species and situations to determine the allocation of carbon to foliage and stems and constrain tree growth patterns. The effects of nutrition are incorporated through the carbon allocation procedure; the amount of carbon allocated below ground will increase with decreasing soil fertility. Recently acquired knowledge about the physiological factors causing decline in forest growth rates with age is used to model that decline. Changes in stem populations (self-thinning) are derived from a procedure based on the -3/2 power law, combined with stem growth rates. The model requires weather data as input, works on monthly time steps and has been run for periods up to 120 years, producing realistic patterns of stem growth and stern diameter increments. The time course of leaf area index is realistic for a range of soil conditions and atmospheric constraints. 3-PG can be run from remotely-sensed estimates of leaf area index coupled to weather data and basic, readily available information about soils and stand characteristics. It is being tested as a practical tool against forestry data from New South Wales, Tasmania, Victoria and New Zealand. Test results show excellent correspondence between stand growth measurements and simulated stem growth over 30 years.